While various schemes have been devised, and trial systems advanced over the past decade, or so, in achieving polarization control for coherent optical communication or other polarization-dependent fiber-optic systems, a scheme which is entirely free of manual adjustment as required in any practical application did not exist until the invention of the U.S. Ser. No. 07/310,872 entitled "Passive Fiber-Optic Polarization Control now U.S. Pat. No. 4,953,132.
In this invention, there was described a new all-passive polarization control scheme which is entirely manual-free, and hence immediately applicable to practical systems requiring a well-defined linear polarization orientation. A method of fabricating the invented passive polarization-control device was exposed, which is to variably spin the preform of appropriate birefringence with a spinning speed droping slowly and monotonously from a sufficiently high value to zero.
The present continuation-in-part application of the U.S. Ser. No. 07/310,872, now U.S. Pat. No. 4,953,132 represents an alternative technological approach to the fabrication of the invented all-passive polarization-control device. Peculiar to this approach is to use a length of birefringent optical fiber, not a specialized preform, as the starting substance, and to make the polarization-control element by locally heating and spinning the fiber with a variable speed which is sufficiently high initially, and which drops slowly and monotonously to zero while the micro-heater moves linearly along the length of the optical fiber.
Besides the dominating advantage of manual-free operation, the invented fiber-optic element/device is advantageous in small size and light weight, in structural simplicity and operational dependability, in low-cost, among others. The price paid for all these advantages is a power penalty of 3 dB. From the power-budget point of view, this generally will not pose a problem, as coherent optical communication systems are capable of improving the sensitivity of the present-day direct-detection receiver by 15-20 dB. In view of the current line of though that coherent optical systems may prove useful also in local networks, where the primary interest is in the multiplexing possibility, while providing a useful increase in power-budget, the present invention adopting the passive-device approach is apparently superior.